You are not doing stage 1 and stage 2 installations anymore (which is sad - they are the funniest parts of installing gentoo).

More power to you that all packages in Ubuntu are compiled exactly the way you want them to be. Personally I must say that my needs are quite different, so none of the 'buntus fit me. I'd prefer LFS but it's a PITA to maintain, so I use gentoo (though Source Mage and FreeBSD were close competitors before I chose gentoo back in ... hmm.. distant past).

Since all packages in *buntu are compiled 100% according your taste you are not missing out on anything. Consider yourself lucky.

EDIT: The computer is not "useless" while compiling. You can do all kind of stuff while the machine is compiling the packages in the background. You know, these days OS'es support multitasking. Your knowledge of gentoo is severely outdated.